Distance measuring apparatus are known which measure distance as a function of time of travel of a measurement light pulse and operate in different ways. All of these apparatuses produce a start signal, related to the time of transmission of the measuring light pulse, for initiating transit time measurement. In some case, the electric signal which triggers the transmitter which generates the light pulse is used to initiate a start signal for a time measuring unit of the apparatus. A problem arises where the light pulse transmitter is a laser diode because it reacts to its trigger pulse with a delay which is not constant.
Additionally, when laser diodes are triggered by high amplitude current pulses for producing the electromagnetic measuring light pulses interference signals are produced which if occurring when the start signal is also produced will shift in time the start signal. The extent of this shift changes in an unpredictable manner. This adversely affects the accuracy of the measurement as a too early or belated start of the time measurement causes the measured time of travel to be greater or less than the actual time of travel so that larger or smaller distance values are obtained. The delays by the transmitter in reacting to its trigger pulse must also be taken into account.
In dealing with the foregoing problem one approach has been to split a part of the generated light pulse and to supply that part as a reference light pulse along a short reference light path within the apparatus to an auxiliary receiver therein having a receiving channel of the same design as the main receiving channel which receives and processes reflected measuring light pulse from the target. The output signal generated by the auxiliary receiver upon reception of the reference light pulse is used to produce a start signal for starting the transit time measurement, whereas the main receiving channel produces in reaction to receipt of a reflected measuring pulse a stop signal for stopping the transit time measurement.
The use of two separate receivers with their separate receiving channels, because of their inherent different signal delays arising from, for example, delays in response time of circuit components in the receiver, reduce the accuracy of the distance measuring results. Also use of two receiver systems increases complexity of the apparatus because fluctuations in response must be minimized as well as the differences in signal delays in the receiver.
Because it is impossible to eliminate differences in the time delays occurring when using two receiving channels with different delays (also drift and operational fuctuation) it is necessary after each transit time measurement to determine a correction factor by measuring the difference of the two time delays in the two receiving channels as part of a calibration operation and to correct the transit time measured value by means of the correction factor. This undesirably increases the complexity of the apparatus.